1. Field of the Invention
The present invention relates to a phantom for quality control or QC phantom used to perform quality assurance of a radiation imaging system that uses radiation or in particular X-rays.
2. Description of the Related Art
Digital radiography (DR) or in particular a digital radiation imaging system (hereinafter referred to as the “computed radiography (CR) system”) that uses a photostimulable phosphor has been more and more widely used as a method replacing conventionally used radiography based on an intensifying screen and a silver salt film.
In this CR system, radiation that has passed through or has been emitted from a subject is absorbed into a radiation image conversion panel containing a photostimulable phosphor, the absorbed energy is emitted as fluorescence through excitation of the panel using excitation light such as infrared rays, and this fluorescence is read as a digital image through conversion into an electric signal.
With such a CR system, it is possible to obtain a digital image having an abundant amount of information, so that it is possible to carry out sophisticated medical diagnosis, for instance. In addition, it is also possible to transmit/receive the digital image as digital data, so that this system is also effective in remote medical care or the like.
By the way, in order to assure use of an X-ray image with a high degree of reliability, it is required to measure and verify the performance of the CR system. If image quality of the CR system is low, high-reliability image analysis is impossible. This is because an image that is low in reliability and unusable is generated owing to lowering of image contrast quality, for instance.
In order to solve this problem, measurement of parameters of the CR system is performed using a QC phantom. This QC phantom includes various image quality evaluation patterns fixed on a substrate. Those image quality evaluation patterns have characteristics such as sizes, shapes, densities, and compositions, are formed using various materials, are designed so that one or more image quality evaluation items of the CR system can be measured, and are set so that an invariance evaluation of different performance parameters of the CR system can be performed.
That is, first, the QC phantom is irradiated between a radiation source and a radiation image conversion panel. Then, image reading is performed, thereby obtaining a digital image in which the image quality evaluation patterns are captured, and a reproduction image thereof. Following this, a quantitative or visual image evaluation is performed, thereby performing a performance evaluation/image evaluation.
As examples of such a QC phantom, there are known a QC phantom disclosed in JP 01-148241 A that protects a resolving power chart made of tungsten in a methacrylic resin and a QC phantom disclosed in JP 2001-299736 A that uses a photoetching metal that is considered to have less distortion due to heat.
When image quality evaluation patterns made of metallic materials are fixed in a resin like in the case of the QC phantom disclosed in JP 01-148241 A, however, cracking may occur to the image quality evaluation patterns owing to differences in coefficient of thermal expansion between the metals and the resin.
Also, when resin patterns are fixed on a metal, for instance, this results in a situation where even if the coefficient of thermal expansion of the metal itself is small, peeling tends to occur owing to differences in coefficient of thermal expansion between the metal and the resins.
It is of course possible to fix the image quality evaluation patterns using structural members such as screws in order to increase fixation strength. In this case, however, such structural members are also captured at the time of X-ray imaging and therefore this construction is not appropriate.